Cellular repair mechanisms are necessary for the reversal of DNA damage caused by a variety of physical and chemical environmental agents. In the absence of repair, DNA damage leads to increased cell death and mutagenesis. Despite the importance of DNA repair in mammalian cells, a detailed understanding of the mechanisms involved and their genetic regulation has not yet been achieved. This is due in part to the large size and complexity of the mammalian cell chromatin and to the absense of suitable repair deficient cells. We propose to investigate DNA repair in mammalian cells by (1) using the Simian virus 40 (SV40) as a probe for cellular repair processes and (2) isolating and characterizing repair deficient mutants of CV-1 (African green monkey kidney) cells. By studying the repair of ultraviolet light induced damage in SV40, a small, well defined DNA virus, repair processes can be analyzed without the complications which arise in the analysis of the high molecular weight cellular DNA. By measuring the rates of excision repair and replicative bypass in relation to the recovery of viral transcription and DNA replication we can determine the relative importance of these two repair pathways in the functional reactivation of the virus. Furthermore, specific steps in the excision repair process can be defined by isolating and characterizing viral DNA which is in the process of being repaired. We will carry out a number of different selection schemes for isolating repair deficient cells. The study of these mutant cells, again using SV40 as a probe, should allow an analysis of the genetic regulation of cellular DNA repair pathways. Furthermore, by characterizing these mutants on the biochemical level, we should be able to gain a better understanding of enzymes involved in repair. In these studies we are focusing on the repair of DNA damage caused by UV irradiation. However, once this viral system has been developed for the study of UV repair, it could be used for the study of the repair of DNA damage caused by other physical or chemical agents as well.